Overcurrent protection circuit

ABSTRACT

The invention relates to an overcurrent protection circuit comprising an overcurrent trip ( 5 ) and a switching element ( 6 ). A switched current (I) passing through the switching element ( 6 ) can be detected by the overcurrent trip ( 5 ) and the switching element ( 6 ) can be tripped to open if said switched current (I) fulfils a tripping requirement. A current limiter ( 7 ) is series-connected downstream of the switching element ( 6 ).

[0001] The present invention relates to an overcurrent protection circuit, having an overcurrent release and a switching element, in which a switching current flowing through the switching element can be detected by means of the overcurrent release and opening of the switching element can be initiated if the switching current satisfies a tripping condition.

[0002] Overcurrent protection circuits such as these are generally known—in particular in the form of circuit breakers.

[0003] Circuit breakers generally have a thermal overload release and an electromagnetic quick-action release. If the current flowing through the circuit breaker slightly exceeds a rated current, tripping takes place with a time delay, by means of the thermal overload release. In the event of a short circuit, in contrast, when the current rises rapidly, the electromagnetic quick-action release trips the circuit breaker with virtually no delay. The tripping of the circuit breaker has two effects. Firstly, the circuit is opened directly, and secondly a switching mechanism is tripped, so that the circuit is not closed automatically again once the circuit breaker has tripped.

[0004] Despite tripping with virtually no delay, the electromagnetic release does have a reaction time. During the reaction time, the load current rises above the identification current at which the electromagnetic release trips. This results in a large amount of wear due to contact erosion in the contact that is to be opened. If the current rises too rapidly, it is even possible in the extreme for a continuous arc to be struck, which can no longer be quenched and leads to complete destruction of the circuit breaker and to it being impossible to switch off the current.

[0005] The object of the present invention is to provide an overcurrent protection current in which an excessive rise in the current is prevented, even in the event of a short circuit in the load circuit.

[0006] The object is achieved in that a current limiter is connected in series with the switching element.

[0007] If the current limiter has a steep current/voltage characteristic at low voltages and has a flat current/voltage characteristic at high voltages, this results in low power loss during normal operation of the overcurrent protection circuit, and in current limiting at a relative low value in the event of a short circuit.

[0008] In some cases, the current limiter may be in the form of a single component; however, generally it is in the form of a current limiting circuit.

[0009] Examples of a current limiter include the current limiter having two back-to-back series-connected transistors or (alternatively) two back-to-back parallel-connected series circuits, which each have one transistor and one diode.

[0010] Current limiting can also be achieved in an effective manner by the current limiter having two self-commutating transistors, each having a gate contact, a source contact and a drain contact, with the source contacts being connected to one another via a resistor, the source contacts of the transistors being connected to the gate contact of the respective other transistor, and the current limiter being connected in series with the switching element via the drain contacts of the transistors.

[0011] If the transistors are in the form of SiC transistors, the current limiter has a particularly low resistance during normal operation, and can absorb a large amount of energy during short-circuit operation.

[0012] If the overcurrent release, the switching element and the current limiter are arranged in a common housing, this results in the overcurrent protection circuit being particularly compact. Furthermore, the overcurrent protection circuit may then be in the form of a unit which can be wired up in advance.

[0013] Generally, the overcurrent release initiates the opening of the switching element with virtually no delay when a limit current is exceeded. If the current limiter is designed appropriately, it is also possible, however, for the switching element to be opened with a time delay when the limit current is exceeded.

[0014] Further advantages and details can be found in the following description of an exemplary embodiment. In this case, illustrated in outline form:

[0015]FIG. 1 shows a circuit,

[0016]FIG. 2 shows an overcurrent protection circuit,

[0017]FIG. 3 shows a current limiter,

[0018]FIG. 4 shows a current/voltage characteristic, and

[0019]FIGS. 5 and 6 show further current limiters.

[0020] As shown in FIG. 1, a load 1 is connected to a voltage source 4 via an on/off switch 2 and an overcurrent protection circuit 3. The on/off switch 2 can be actuated from the outside by means of an appropriate control signal S. When the on/off switch 2 is closed, a switching current I flows through the overcurrent protection circuit 3 and through the load 1.

[0021] As shown in FIG. 2, the overcurrent protection circuit 3 has an overcurrent release 5, a switching element 6 and a current limiter 7. The switching element 6 is generally closed, so that the switching current I can flow. However, the switching current I, which also flows through the switching element 6, can be detected by means of the overcurrent release 5 and opening of the switching element 6 can be initiated when the switching current I satisfies a tripping condition. As can be seen from FIG. 2, the overcurrent release 5, the switching element 6 and the current limiter 7 are connected in series and are arranged in a common housing 10. The switching current I is in this case limited by means of the current limiter 7.

[0022] The overcurrent release 5 generally has two tripping elements 8, 9, namely an electromagnetic quick-action release 8 and a thermal overload release 9. The opening of the switching element 6 is initiated with virtually no delay by means of the quick-action release 8 when the switching current I exceeds a limit current which is several times a rated current, typically 12 or 19 times the rated current. Opening of the switching element 6 is initiated with a time delay by means of the overload release 9 when the switching current I is above the rated current for a relatively long time, for example being 1.2 times, 1.5 times, twice or 7.2 times the rated current.

[0023] Firstly, the switching element 6 has a switch by means of which the circuit can be opened and closed as such, and secondly has a switching mechanism by means of which the switch can be fixed in its open or closed position. When one of the tripping elements 8, 9 responds, not only is the switch of the switching element 6 opened but the switching mechanism is also released. Once the switching element 6 has been opened, this is thus not automatically closed again.

[0024] The circuit illustrated in FIG. 2 is generally used. In this case, in the event of a short circuit, initiation of the opening of the switching element 6 is a possible with virtually no delay.

[0025] If, in contrast, the current limiter 7 is appropriately designed such that it copes with a short circuit until the thermal overload release 9 responds, there is no need for the quick-action release 8. This is indicated in FIG. 2 by the quick-action release 8 being shown only by dashed lines. Thus, in this case, the opening of the switching element 6 is initiated with a time delay even in the event of a short circuit.

[0026] By way of example, FIG. 3 now shows one possible refinement of the current limiter 7. As shown in FIG. 3, the current limiter 7 has two self-commutating transistors 11, 12. In principle, the transistors 11, 12 may be of any desired nature. In particular, they may be in the form of MOSFETs, IGBTs or else bipolar transistors. According to the exemplary embodiment, they are in the form of SiC field-effect transistors 11, 12. Each of the field-effect transistors 11, 12 has one gate contact 13, one source contact 14 and one drain contact 15. The source contacts 14 are connected to one another via a variable resistor 16. Furthermore, the source contacts 14 of the field-effect transistors 11, 12 are connected to the gate contact 13 of the respective other field-effect transistor 12, 11. The current limiter 7 is then connected in series with the switching element 6 via the drain contracts 15.

[0027] The current limiter shown in FIG. 3 may be formed from discrete components 11, 12, 16. However, as is indicated by a dashed-dotted line in FIG. 3, it may also be in the form of a monolithically integrated circuit.

[0028] The current limiter 7 illustrated in FIG. 3 has a current/voltage characteristic as shown in FIG. 4. As can be seen from FIG. 4, the characteristic has a steep profile at low voltages and has a flat profile at high voltages. In particular, the switching current I is limited to the maximum value shown by the dashed line, even at high voltages U. The maximum value can be adjusted by appropriate adjustment of the resistor 16.

[0029] Alternatively, the transistors 11, 12 may also be connected back-to-back in series as shown in FIG. 5. In this case, a diode 17, 18 is preferably connected in parallel with each of them. In the further embodiment which is illustrated in FIG. 6, the current limiter 7 has back-to-back parallel-connected series circuits. Each series circuit has a transistor 11 or 12, respectively, with which a diode 17, 18 is connected in series. The transistors 11, 12 are also in the form of SiC components in the embodiments shown in FIGS. 5 and 6.

[0030] The overcurrent protection circuit 7 according to the invention is particularly suitable for use in AC voltage circuits. However, in principle, it can also be used in DC voltage circuits. 

1. An overcurrent protection circuit, having an overcurrent release (5) and a switching element (6), in which a switching current (I) flowing through the switching element (6) can be detected by means of the overcurrent release (5) and opening of the switching element (6) can be initiated if the switching current (I) satisfies a tripping condition, characterized in that the switching element (6) is connected in series with a current limiter (7).
 2. The overcurrent protection circuit as claimed in claim 1, characterized in that the current limiter (7) has a steep current/voltage characteristic at low voltages (U), and has a flat current/voltage characteristic at high voltages (U).
 3. The overcurrent protection circuit as claimed in claim 1 or 2, characterized in that the current limiter (7) has two back-to-back series-connected transistors (11, 12).
 4. The overcurrent protection circuit as claimed in claim 1 or 2, characterized in that the current limiter (7) has two back-to-back parallel-connected series circuits, which each have one transistor (11, 12) and one diode (17, 18).
 5. The overcurrent protection circuit as claimed in claim 1 or 2, characterized in that the current limiter (7) has two self-commutating transistors (11, 12) each having a gate contact (14), a source contact (14) and a drain contact (15), in that the source contacts (14) are connected to one another via a resistor (16), in that the source contacts (14) of the transistors (11, 12) are connected to the gate contact (13) of the respective other transistor (12, 11) and in that the current limiter (7) is connected in series with the switching element (6) via the drain contracts (15) of the transistors (11, 12).
 6. The overcurrent protection circuit as claimed in claim 3, 4 or 5, characterized in that the transistors (11, 12) are in the form of SiC transistors (11, 12).
 7. The overcurrent protection circuit as claimed in one of the above claims, characterized in that the current limiter (7) is in the form of a monolithically integrated circuit (7).
 8. The overcurrent protection circuit as claimed in one of the above claims, characterized in that the overcurrent release (5), the switching element (6) and the current limiter (7) are arranged in a common housing (10).
 9. The overcurrent protection circuit as claimed in claims 1 to 8, characterized in that the overcurrent release (5) initiates the opening of the switching element (6) with virtually no delay when the switching current (I) exceeds a limit current.
 10. The overcurrent protection circuit as claimed in one of claims 1 to 8, characterized in that the overcurrent release (5) initiates the opening of the switching element (6) with a time delay when the switching current (I) exceeds a limit current. 